1. Field of the Invention
The present invention relates to printed circuit boards and more particularly to a method and apparatus for applying solder to the contact points or terminal pads on a circuit board which are to be electrically and mechanically connected to circuit components.
2. Description of the Prior Art
The electronics industry has become increasingly dependent on printed circuits for the interconnection of electronic components. Initially the electronic components were secured to the boards via leads which extended through holes in the boards. Today many components such as integrated circuits ("ICs") are mounted directly to the surface of the boards, i.e. by soldering the leads directly to terminal pads on the boards.
Technological improvements in integrated circuits have resulted in increased terminal lead densities with the attendant requirement of increased terminal pad densities on the printed circuit boards. The distance between the leads (i.e. pitch) of integrated circuits of the order of 0.015 inches (i.e. 15 mils) are not uncommon. Such a density requires that the spacing between the edges of the circuit board terminal pads be of the order of 7.5 mils.
To accommodate surface mounted components, such as ICs, on circuit boards, it is necessary to accurately apply solderable material such as a tin/lead alloy ("solder") or a solder containing paste to the circuit board terminal pads prior to the mounting operation. Once the solder or solder paste has been applied to the terminal pads, the surface mounted components can then be secured on the board by a suitable adhesive or the paste itself with the leads in contact with the terminal pads. The assembly is then heated by means of an oven heat lamp etc. to a temperature sufficient to melt the solder to form a electrical and mechanical bond between the terminal pads and the component leads.
One method commonly used to apply solder to the terminal pads of printed circuit boards is called dip soldering. In accordance with this method, the circuit board is dipped into a bath of molten solder and removed so that the metalized areas of the board not provided (e.g. terminal pads) receive a layer of solder. However, due to the solder's surface tension, the resulting layer is generally dome shaped as is illustrated in FIG. 1 of the drawings. Streams of hot air may be used to remove some of the excess solder. However, the depth of the solder on the terminal pads will vary. When surface mounted components, such as IC's with high density lead terminals are mounted on the board and the solder heated, some of the lead terminals may not be wetted by the molten solder because of the uneven height of the solder on the pads. While the open circuits resulting from the insufficient depth of solder may be correctable, the time and cost of finding and remedying the open circuits is significant.
Another method commonly used to apply solder to terminal pads involves the use of a solder paste which is deposited on the terminal pads by silk screening or mask printing techniques. The surface mounted components are then placed on the board so that their lead terminals are embedded in the paste on the terminal pads. The components are held in place by the surface tension of the paste. The assembly is then heated to melt the solder and evaporate the volatile components and solvents of the paste. The paste, however, is expensive and difficult to apply. In addition, the paste may not accurately follow the contours of the pads resulting in an unacceptable finished product.
More recently two additional methods of applying solder to the terminals pads of printed circuit boards have been disclosed in the prior art, German Patent No. DE 3,810,653 which issued to Dieter & Gretta Freidrich on May 18, 1988, teaches a method for depositing solder on the terminal pads on one side of a circuit board in which a sacrificial solder resist mask is used to border the terminal pads and define the height of the solder. In accordance with the Friedrich patent, the board with the sacrificial mask in place is dipped into a bath of molten solder and a flat covering device, such as a plate, is then clamped against the board to hold the solder in place and within the cavities on the board formed by the boarding mask and the terminal pads. The board and covering device are lifted as a unit from the solder bath and as soon as the solder solidifies, the covering device is removed. The sacrificial mask is then stripped from the board leaving a layer of solder on each of the terminal pads. While this method may deposit a uniform layer of solder on the pads it is limited to relatively small circuit boards and not amenable to mass production.
U.S. Pat. No. 4,720,324, which issued to John S. Hayward on Jan. 19, 1988, discloses a method in which a layer of a tin/lead alloy is electroplated onto the terminal pad areas and subsequently heated and amalgamated into solder in the presence of flux to wet the leads of the surface mounted components. The '324 process requires several complex electroplating, stripping and etching steps which would add considerably to the cost of the resulting assembly.
There is a need for a simple and reliable method and apparatus for applying a uniform layer of solder to the conductive terminal pads of printed circuit boards which is amenable to mass production techniques.